Noise is one major cause of reducing vehicle ride comfort. There are many different kinds of noises generated in a motor vehicle, including a noise of an air conditioner. The noise of an air conditioner is generated in a process of compressing a coolant in a compressor. If the noise is introduced into a vehicle room, vehicle ride comfort is significantly reduced.
In particular, a vibration and a noise generated in a compressor are transmitted into a vehicle room along a coolant and a coolant pipe. Due to the vibration and the noise thus transmitted, a vehicle occupant residing within a vehicle room receives a severe stress and feels fatigue. Thus, the vibration and the noise reduce vehicle ride comfort and significantly hinder safe driving. Accordingly, the noise generated from the compressor of the vehicle air conditioning system needs to be reduced in order to improve vehicle ride comfort and to assure safe driving.
As a method of reducing a noise generated in a compressor, there is available a technology in which a silencer is installed in a coolant pipe connected to the compressor.
As shown in FIG. 1, this technology makes use of a Helmholtz resonance principle and has a structure in which a resonance tube 3 is installed within a coolant pipe 1. The resonance tube 3 includes a coolant introduction portion 5, a neck portion 7 whose diameter gradually decreases from the coolant introduction portion 5, and a coolant discharge portion 9 whose diameter gradually increases from the neck portion 7. A resonance chamber 7a is defined between the outer circumferential surface of the neck portion 7 and the inner circumferential surface of the coolant pipe 1.
In this resonance tube 3, during a process of introducing and discharging a coolant gas having a high temperature and a high pressure, a vibration and a noise is absorbed into the resonance chamber 7a through a resonance hole 7b formed in the neck portion 7. As a result, the vibration and the noise generated in the compressor are prevented from being transmitted into a vehicle room through the coolant and the coolant pipe. This makes it possible to prevent deterioration of vehicle ride comfort which may be caused by the vibration and the noise generated in the compressor.
In the conventional air conditioning system mentioned above, the coolant gas having a high temperature and a high pressure should pass through the resonance tube 3 having a small diameter. This leads to a sharp decrease in the coolant fluidity, which in turn results in a decrease in the vehicle room cooling performance.
Oil for lubricating the compressor is contained in the coolant gas. As shown in FIG. 2, the oil passing through the neck portion 7 of the resonance tube 3 is introduced through the resonance hole 7b and is stagnated in the resonance chamber 7a as indicated by “A”. This reduces the performance of the compressor and significantly reduces the noise absorption performance of the resonance chamber 7a. 
Particularly, oil shortage occurs in the compressor due to the stagnation of the oil in the resonance chamber 7a. This may drastically reduce the compressor performance and may cause trouble in the compressor.
The stagnation of the oil in the resonance chamber 7a leads to a decrease in the effective volume of the resonance chamber 7a and a sharp reduction in the noise absorption performance of the resonance chamber 7a. As a consequence, the efficiency of reducing the noise and the vibration of the compressor is significantly reduced.